Treatment of slags covering molten metals

ABSTRACT

Molten slag layers on molten metal may be removed by mixing it with pumice to coagulate it and removing the so coagulated semisolid slag.

United States it [191 Philpotts TREATMENT OF SLAGS COVERING MOLTEN METALS [75] Inventor: Andrew Philpotts, Nechells,

England [73] Assignee: Foseco International Limited,

Birmingham, England [22] Filed: May 22, 1972 [21] Appl. No.: 255,378

Related U.S. Application Data [63] Continuation of Ser. No. 861,476, Sept. 26, 1969,

abandoned.

[30] lForeign Application Priority Data 7 [51] Int. Cl. C2lb 3/04, C210 7/00, C22b 9/10 [58] Field of Search 75/24, 94, 93, 53

[ Nov. 19, 1974 [56] References Cited UNITED STATES PATENTS 120,099 10/1871 Quann 75/94 222,369 12/1879 Tate 75/94 429,777 6/1890 Severance... 148/26 1,599,056 9/1926 Lloyd 148/26 3,423,202 1/1969 Ledune 75/53 FOREIGN PATENTS OR APPLICATIONS 979,583 1/1965 Great Britain 75/10 R Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Wolfe, Hubbard, Leydig, Voit & Osann, Ltd.

[ 5 7] ABSTRACT Molten slag layers on molten metal may be removed by mixing it with pumice to coagulate it and removing the so coagulated semi-solid slag.

9 Claims, No Drawings TREATMENT OF SLAGS COVERING MOLTIEN METALS This application is a continuation of application Ser. No. 861,476, filed Sept. 26, 1969 now abandoned.

This invention relates to the treatment of slags covering molten metal. It is of general value irrespective of the nature of the molten metal and accordingly of the slag associated with it, i.e. the invention is of value both in the case of slags covering steel or other ferrous metals and in the case of slags covering non-ferrous metals.

In metallurgical practice there are various circumstances which may lead to a situation in which a body of molten metal is contained in a vessel, e.g. a ladle or electric melting unit, and is surmounted by a supernatant slag layer. Where, as is often the case, the slag is of relatively low viscosity it may present serious practical difficulties. Thus it will usually be difficult to remove, because of its low viscosity, and it will tend to wet, and so erode, a refractory lining present in the vessel. Further it may tend to become partially intermixed with the molten metal so that droplets of the slag become trapped in the molten metal and remain as inclusions in the metal when the metal is later cast to solid ingot or cast shapes. The problem has been noted to be particularly severe when desulphurising agents have been added to molten ferrous alloys, the desulphurising agent existing essentially in the slag.

Because of the difficulty of removing a slag of low viscosity the attempt is sometimes made to increase the slag viscosity by the addition of sand. This method is not without its disadvantages because the sand used may well contain impurities which will enter the molten metal and adversely affect its properties. Moreover sand is not a very effective agent for coagulating the slags and it does not assist very much, if at all, in reducing the tendency ofthe slag to erode the refractory lin- -ing of the molten-metal-containing vessel. It has now been found that remarkable and unexpected advantages may be achieved in the handling of such slags by the addition thereto of the mineral, pumice.

According to the present invention therefore, a process for the removal of slag covering molten metal comprises incorporating in the said slag the mineral pumice and removing the resultant coagulated slag from the surface of the molten metal.

Pumice is a mineral consisting primarily of silica and alumina with small quantities of other metal oxides such as CaO, MgO, F6203, Na O, K20.

I it is of volcanic origin and occurs naturally as a low density (usually less than 1 gm/cc) material, which may have a rigid cellular or spongy structure. At the heat of molten metal pumice generally disintegrates to granular or powdered form, but it is preferable to use the pumice in already disintegrated form. e.g. in the range minus l+60 885 mesh. Powdered pumice in this particle size range generally has a bulk density between 0.54 and 0.64 gm/cc. It is found that by the addition of pumice to a slag as aforesaid the slag is coagulated or thickened so that the slag may be skimmed off the molten metal very simply. The mechanism involved is not understood but the effect is to cause the slag to become converted to a pancake-like cover of rubbery consistency. In such form either the slag can be readily removed from the surface or the molten metal may be poured from beneath it with minimum risk of contamination of the molten metal. Furthermore the pumice appears to act as a scavenger for and collector of free droplets of the slag, thus reducing risk of metal contamination during casting.

The pumice may be employed as such in cases where the temperature of the molten metal is high enough to melt the pumice but it is often desirable to include the pumice in an additive composition which has, by reason of the presence of substances other than the pumice a melting point or range not greatly below the temperature of the cast metal. Where slags on nickel alloys are treated it is usually convenient to use pumice without any additions. Where slags on metals at much higher temperatures, e.g. steel, are concerned it is usually desirable to use the pumice as a pumice composition containing one or more substances to increase the melting point or range, e.g. china clay. Where slags on metals at lower temperatures eg copper or copper base alloys are concerned it is usually desirable to use the pumice as a pumice composition containing iron oxide or powdered glass.

The pumice or pumice composition may be added to the vessel before the molten metal is poured into the vessel or may be added during or after the pouring of the molten metal. It is often advantageous to add the pumice or pumice composition while the molten metal is being poured as this tends to increase the scavenging and cleaning effect of the pumice. The optimum conditions will, however, vary with the particular metal under treatment. In the case of copper base alloys it is generally less desirable to add the pumice to the vessel before the metal is poured into the vessel, though this technique is quite satisfactory when dealing with molten cast iron. Whichever method is employed it is usually only necessary to wait for a very short period, e.g. time enough for all the slag to rise to the surface, often only a few seconds, and the treated slag can then be re- .moved or the molten metal poured from beneath it.

The optimum quantity of pumice used will also depend on the particular metal and temperature conditions, and the nature and quality of the slag, and it is therefore only possible to afford a general indication of desirable quantities. For cast iron, a rate of 0.22 to 0.45 g per Kg of metal is usually required. Suitable quantities for most practical purposes are indicated in the following examples.

The following examples will serve to illustrate the invention.

EXAMPLE I An additive of the following composition is prepared:

By weight Powdered pumice 50% Powdered glass 25% Sodium carbonate 257:

This additive is of particular value for the treatment of slags on copper base alloys and the following are the recommended procedures for use as applied to copper base alloy in lOO lbs. Kg) crucible.

1. add 7-21 g. of the additive, rabble until dry and skim off. Place afurther 7-14 g. on the metal surface particularly with a view to forming a dam or bridge across the ladle lip. Allow to crust for a few moments before pouring. Do not break the crust formed;

2. when tapping into carrying ladles, add 7-21 g. of the additive to the metal stream. Rabble in when the pot is full and skim clean. Add a further 7-14 g. as described under (1) 3. where the metal is melted under a dry cover, e.g. charcoal, it is not necessary to skim completely. The cover should be pushed back from the lip and 7-21 g. of the additive sprinkled on to the cleared metal so as to form a floating dam across the lip. This will effectively hold back charcoal, etc. no matter how many times the crucible is tilted, provided it is not broken.

EXAMPLE 2 An additive of the following composition is prepared:

By weight Powdered pumice 91% Sodium carbonate 57: iron oxide (Hacmatitc) 3'72 Graphite 1% This additive is of particular value for the treatment of slags on cast irons for example, grade 11/14 cast iron. It is preferred to employ 0.22 to 0.45 g. of the additive per Kg. of iron. it may be added to the empty or partly filled ladle or sprinkled on the molten iron surface, whichever may be more convenient.

1n the treatment of slags on other molten metals, other compositions and quantities may be desired but are readily formulated, the criteria being that the pumice should be added as such or as a composition which becomes molten at the temperature of the molten metal but which tends to solidify during the normal casting of the molten metal in the treatment vessel, e.g. ladle, and that it should be added in a quantity sufficient to form with the slag a semi-solid or rubbery prod- LlCt.

The use of pumice for the purpose indicated leads to important advantages, some of which have already been briefly mentioned. Particular advantages include the following:

a. There is no or very little contamination of the metal by the use of pumice in the manner indicated since pumice contains no metallic elements which are likely to alter the properties of the molten metal or of solid metal cast from it.

b. The pumice cleans the metal and takes impurities to the top and holds them in the slag, thus reducing any possibility of slag being poured into the mould or ingot when the metal is low in the ladle or like vessel.

c. Pumice has a high insulating property and tends to maintain the temperature for a longer period than is normally experienced.

d. Pumice tends to attract and remove the slag and foreign matter from the walls of the vessel thus cleaning them and improving their life. Furthermore, repeated use of pumice in this way gradually causes the formation ofa vitreous coating on the refractory lining which is resistant to attack of slags.

e. When placed in the bottom of a vessel before charging the vessel with molten metal, the material gives a gentle mixing action during the time in which the vessel is being filled.

f. When used with molten iron after desulphurisation, the resulting coagulated slag tends to prevent resulphurisation a phenomenon which is experienced from time to time. it is believed that this property of preventing resulphurisation may be due to the fact that the residue from the desulphurising agent is trapped with the coagulated slag and held on the surface of the metal. This prevents reduction of the sulphur to a form where it can re-enter the metal.

g. There are no obnoxious fumes or smoke generated by the addition of pumice which creates a heavy slag, adherent to itself and acting as an insulator on the surface of the metal.

h. Pumice is economical to use when first introduced, between a quarter and one half pound may be sprinkled on the surface of the skimmed metal and a pancake-like cover is formed. Metal may be poured from under this cover on tilting the ladle. After pouring the metal from the ladle the coagulated slag may be left in the ladle and fresh metal tapped on to it.

It is to be observed that the results described herein as obtainable by the use of pumice are not obtainable, or only to a less satisfactory extent, by other materials; thus even the substance perlite, which is closely related in a chemical sence, is less suitable particularly because it contains combined water which at the temperature of molten metal is converted to steam which may cause gas occulsions in the metal. Even apart from this difficulty pumice has the advantage over perlite in affecting good results at only half the added weight, and of having a higher fusion point.

I claim as my invention:

1. In a process for separating slag covering molten metal from the molten metal, the improvement comprising incorporating in said slag an additive consisting essentially of pumice in an amount to coagulate and sufficiently thicken said slag into a semi-solid mass capable of being separated from the molten metal without any significant inclusion of the slag therein and thereafter separating the resultant thickened and coagulated, semi-solid slag from the surface of the metal.

2. A process according to claim 1 wherein the molten metal is nickel.

3. A process according to claim 1 wherein the said substance is china clay.

4. A process according to claim 1 wherein the molten metal is selected from the group consisting of copper and copper-based alloys and the pumice additive includes a material selected the class consisting of iron oxide and powdered glass.

5. A process according to claim 1 wherein the quantity of additive comprising pumice which is added to the slag is sufficient to form with the slag a semi-solid rubbery product.

6. A process according to claim 1 wherein the quantity of additive containing pumice which is added to the slag is 0.22 to 0.45 g per Kg of molten metal.

7. A process according to claim 1 wherein the molten metal is steel and an additive is incorporated in said slag which causes the pumice to be molten at the temperature of the molten steel.

8. A composition for use in the process of claim 1 to allow separation of slag covering molten metal from the molten metal which consists essentially of pumice in a powdered condition and in an amount sufficient to cause the slag to coagulate and thicken into a semisolid mass capable of being separated from the molten metal without any significant inclusion of the slag therein and a substance selected from the group consisting of china clay, iron oxide and powdered glass, the substance being selected and being present in an amount sufficient to cause the pumice to be molten at the temperature of the molten metal.

9. A process for treating slag covering molten metal in a vessel to coagulate and thicken the slag and facilitate its removal from the molten metal which comprises incorporating a composition consisting essentially of' pumice and a material selected from the group consisting of china clay, iron oxide and powdered glass into the slag, the pumice being present in an amount suffisolid slag from the surface of the molten metal. 

1. IN A PROCESS FOR SEPARATING SLAG COVERING MOLTEN METAL FROM THE MOLTEN METAL, THE IMPROVEMENT COMPRISING INCORPORATING IN SAID SLAG AN ADDITIVE CONSISTING ESSENTIALLY OF PUMICE IN AN AMOUNT TO COAGULATE AND SUFFICIENTLY THICKEN SAID SLAG INTO A SEMI-SOLID MASS CAPABLE OF BEING SEPARATED FROM THE MOLTEN METAL WITHOUT ANY SIGNIFICANT INCLUSION OF THE SLAG THEREIN AND THERAFTER SEPARATING THE RESULTANT THICKENED AND COAGULATED, SEMI-SOLID SLAG FROM THE SURFACE OF THE METAL.
 2. A process according to claim 1 wherein the molten metal is nickel.
 3. A process according to claim 1 wherein the said substance is china clay.
 4. A process according to claim 1 wherein the molten metal is selected from the group consisting of copper and copper-based alloys and the pumice additive includes a material selected the class consisting of iron oxide and powdered glass.
 5. A process according to claim 1 wherein the quantity of additive comprising pumice which is added to the slag is sufficient to form with the slag a semi-solid rubbery product.
 6. A process according to claim 1 wherein the quantity of additive containing pumice which is added to the slag is 0.22 to 0.45 g per Kg of molten metal.
 7. A process according to claim 1 wherein the molten metal is steel and an additive is incorporated in said slag which causes the pumice to be molten at the temperature of the molten steel.
 8. A composition for use in the process of claim 1 to allow separation of slag covering molten metal from the molten metal which consists essentially of pumice in a powdered condition and in an amount sufficient to cause the slag to coagulate and thicken into a semi-solid mass capable of being separated from the molten metal without any significant inclusion of the slag therein and a substance selected from the group consisting of china clay, iron oxide and powdered glass, the substance being selected and being present in an amount sufficient to cause the pumice to be molten at the temperature of the molten metal.
 9. A process for treating slag covering molten metal in a vessel to coagulate and thicken the slag and facilitate its removal from the molten metal which comprises incorporating a composition consisting essentially of pumice and a material selected from the group consisting of china clay, iron oxide and powdered glass into the slag, the pumice being present in an amount sufficient to coagulate and thicken the slag into a semi-solid mass capable of being separated from the molten metal without any significant inclusion of the slag therein and the material being selected and being present in an amount sufficient to cause the composition to become molten at the temperature of the molten metal and separating the resultant coagulated and thickened, semi-solid slag from the surface of the molten metal. 